In PET imaging, positrons are emitted from a radiopharmaceutically doped organ or tissue mass of interest. The positrons combine with electrons and are annihilated and, in general, two gamma photons which travel in diametrically opposite directions are generated upon that annihilation. Opposing crystal detectors, which each scintillate upon being struck by a gamma photon, are used to detect the emitted gamma photons. By identifying the location of each of two essentially simultaneous gamma interactions as evidenced by two essentially simultaneous scintillation events, a line in space along which the two gamma photons have traveled (a “line of response,” or “LOR”) can be determined. The LORs associated with many million gamma interactions with the detectors are calculated and “composited” to generate an image of the organ or tissue mass of interest, as is known in the art.
In some of the earlier PET systems, the gamma detectors could be used only to determine the location of gamma interaction with the detector in two dimensions, which gave rise to parallax errors. More particularly, a conventional two-dimensional measurement of the spatial location of a detected gamma ray absorption event in the scintillating crystal is limited to a two-dimensional point in the X,Y plane of the crystal. However, because the number of scintillation photons reaching each detector element (e.g., either a PMT or a photodiode) is dependent on the solid angle subtended by the area of that detector element to the point of the gamma ray absorption within the crystal, the amount of scintillation photons received by each detector is also a function of the depth of interaction (DOI) of the incident gamma ray within the crystal, i.e., along the Z axis of the crystal.
The DOI is an important parameter when applied to imaging detector geometries where the directions from which incident gamma rays impinge upon the crystal are not all substantially normal to the crystal surface. If incident gamma rays intersect the crystal from directions not normal to the crystal, the unknown depth of interaction of those gamma rays within the crystal will result in an additional uncertainty in the measured position of the interaction because of the parallax effect, if only a two dimensional (i.e., X,Y) spatial location is calculated for such an absorption event. A detailed explanation of the importance of and the problems associated with the DOI is provided in “Maximum Likelihood Positioning in the Scintillation Camera Using Depth of Interaction,” D. Gagnon et al., IEEE Transactions on Medical Imaging, Vol. 12, No. 1, March 1993, pp. 101-107.
Thus, parallax errors could be reduced by using depth of interaction (DOI) information to increase the spatial resolution of the system, i.e., to provide the location of gamma interaction in three dimensions in space. In this regard, some research brain PET scanners are able to provide DOI information using so-called “phoswich” (for “phosphorescence sandwich”) detectors, constructed as axially stacked scintillators, using a pulse shape discrimination method to minimize parallax error as disclosed in U.S. Pat. No. 6,288,399 to Andreaco et al.
The articles J. Vaquero et al., “A depth-encoding PET detector module with improved spatial sampling”. 1998 Conf. Rec. IEEE NSS and MC Conf. M6-29 and F. Cayouette et al., “Monte Carlo simulation using Detect 2000 of a multilayered scintillation block and fit to experimental data”. IEEE Trans. Nucl. Sci. 50, 339-343, 2003 describe another solution for measuring DOI. Their approach describes the use of two layers of crystal arrays that are offset in the x and y direction. This allows the identification of depth by assigning the events to one of the layers with the superimposed, overall crystal map. This concept is often used in combination with multi-anode PMTs, because a large number of channels is required to achieve the necessary position resolution
US Patent Application No. 2007/0090298 by Shao and the article W. Moses and S. Derenzo, “Design studies for a PET detector module using a PIN photodiode to measure depth of interaction”. IEEE Trans. Nucl. Sci. 41, 1441-1445, 1994 describe still another approach for measuring DOI. In their approach, detectors with dual-ended readout of the scintillator array are used. This is particularly advantageous in combination with thin photosensors such as PIN photodiodes, avalanche photodiodes (APDs) or silicon photomultipliers (SiPMs). Those thin photosensors can be coupled to the scintillator even on the side facing the incident radiation, without leading to high absorption losses and without using much space. DOI information is then obtained by analysing the ratio of signals read out at either end of the scintillator.
The use of multiple, stacked detector modules, each consisting of a scintillator block read out by a number of photosensors is described in G. Llosa, “Experimental results and applications of FBK-irst SiPM pixels and matrices by the DASIPM collaboration”, NDIP08 Conference Talk. This is approach is used in combination with thin photosensors such as SiPMs, that do not occupy much space in between the scintillator layers.
Lewellen et al. have proposed an approach in “DMice—a depth-of-interaction detector design for PET scanners”, 2004 Conf. Rec. IEEE NSS and MIC, and in U.S. Patent Application No. 2009/0224164, to obtain DOI information by measuring the degree of light sharing for pairs of crystals with a systematically varied optical coupling along the common interface. Detector blocks are then assembled from a multitude of such pairs. Their read-out relies on a one-on-one coupling scheme, where each of the crystals in a pair is co-registered with the anode pad of a multi-anode PMT or a SiPM read-out channel. The result is an encoding of the DOI information in the one-dimensional light sharing profile within each crystal pair.
However, each of these approaches have deficiencies such as high cost, difficulty of implementation, need for additional equipment, and the like.
There remains a need in the art, however, for further improvement in the light collection efficiency and spatial resolution of such a DOI scintillation detector.